1. Field of the Invention
The present invention relates to a solid oxide fuel cell, more particularly to a distributed power source and to a solid oxide fuel cell which is used in a cogeneration system or the like and which is suitable for internal reforming solid oxide fuel cell.
2. Description of the Related Art
The solid oxide fuel cell (hereinafter referred to as xe2x80x9cSOFCxe2x80x9d) is a fuel cell in which an oxide ion conductive solid electrolyte is used as an electrolyte. Because, for SOFC, the electrolyte is solid, there is no problem of its dissipation and long life can be expected. Moreover, because the operating temperature which is about 1000xc2x0 C. is high, the utility value of waste heat is high. Furthermore, because output power density is high, it can also be expected to be compact and of high efficiency.
FIG. 7 shows the schematic block diagram of SOFC. In FIG. 7, SOFC 10 has the structure that a fuel electrode 14 and an air electrode 16 are connected to both faces of an electrolyte 12 to give an electrolyte electrode assembly 18 and that further both the sides are put between gas separators 20 and 22. In general, the oxygen ion conductive solid electrolyte such as yttria stabilized zirconia (hereinafter referred to as xe2x80x9cYSZxe2x80x9d) is used in the electrolyte 12. Furthermore, generally, a cermet (hereinafter referred to as xe2x80x9cNi-8YSZxe2x80x9d) of nickel and YSZ containing 8 mol % of yttria (Y2O3) is used in a fuel electrode 14 and a complex oxide such as lanthanum strontium manganate (LaSrMnO3) is used in an air electrode 16.
If hydrogen and air are supplied to the fuel electrode 14 and the air electrode 16 of such SOFC 10, respectively, oxygen becomes an ion in the air electrode 16 and is transported to the fuel electrode 14 side through the electrolyte 12 because there is a difference between the oxygen partial pressure P1 on the air electrode 16 side and oxygen partial pressure P2 on the fuel electrode 14 side. Moreover, an oxide ion reaching the fuel electrode 14 reacts with hydrogen to produce water with emission of an electron. Therefore, if load 22 is connected to the fuel electrode 14 and the air electrode 16, the free energy change of a cell reaction can be directly taken out as electrical energy.
Then, in SOFC, because of its high operating temperature, it is possible to perform the so-called xe2x80x9cinternal reformingxe2x80x9d in which hydrocarbon is directly supplied to the fuel electrode to be reformed to hydrogen in a cell body, instead of supplying hydrogen to the fuel electrode. An internal reforming type SOFC has an advantage of high heat efficiency since heat generated inside the cell can be utilized for a reforming reaction with great endothermic reaction. In addition, because an external reforming vessel is unnecessary, there is an advantage that a fuel cell system can be compact.
The reforming reaction of hydrocarbon is a reaction through which hydrocarbon and steam are reacted with each other and that ultimately hydrogen and carbon dioxide are generated. That is, in order to cause a reforming reaction, steam is necessary. If the ratio of steam to carbon in hydrocarbon contained in fuel (hereinafter referred to as xe2x80x9cS/C ratioxe2x80x9d) is small, unreacted hydrocarbon is directly decomposed on a high temperature and carbon is deposited on the fuel electrode. If carbon is deposited at the fuel electrode, a catalyst is poisoned, which causes a reduction in the output of SOFC.
On the other hand, in the case of the internal reforming type SOFC, usually methane is used for fuel. This is due to the fact that since methane is a chief component of natural gas, it ensures safety, inexpensiveness, and ease in storage and supply. In general, in the internal reforming type SOFC using methane for fuel, in order to inhibit deposition of carbon and to obtain a stable output, it is said that the S/C ratio of the order of 2 to 3 is necessary (for example, see (1) xe2x80x9cSolid Oxide Fuel Cell And Earth Environmentxe2x80x9d written by Hiroaki Tagawa, AGNE SHOFU PUBLISHING INC., p65, (2) preliminary reports of the fifth symposium lecture on fuel cell, pp.173-177, (3) Abulet Abudula et al., Electrochemistry, vol. 65, No. 10, pp.852-858 (1997)).
The difference between the open circuit voltage of the fuel cell and terminal voltage when actually carrying electric current, that is, the magnitude of overvoltage is generally affected by the ohmic resistance of an electrolyte, reaction resistance of a fuel electrode, that of an air electrode or the like. In the case of the internal reforming type SOFC, in addition, because the reforming reaction in the fuel electrode appears as resistance, a reduced catalytic activity of the fuel electrode against the reforming reaction causes an increase in the resistance of the fuel electrode and a reduction in the output power density and the generation efficiency of SOFC. Thus, to increase the generating performance of the internal reforming type SOFC, it is important to increase the catalytic activity of the fuel electrode.
However, Ni-8YSZ heretofore, generally used as a fuel electrode material has an insufficient catalytic activity against the reforming reaction. Therefore, a fuel electrode material with higher activity suitable for the internal reforming type SOFC is desired.
Moreover, to perform stable internal reforming, as described above, relatively large S/C ratio is necessary. However, excessive addition of steam causes a reduction in the open circuit voltage of SOFC. Furthermore, excessive addition of steam causes a reduction in SOFC efficiency since it increases auxiliary utility power and since spare energy is necessary for steam generation. On the other hand, if the S/C ratio is made small to avoid this, carbon could be deposited and thus poison the catalyst.
Furthermore, when Ni-8YSZ is used as the fuel electrode of the internal reforming type SOFC, its initial performance is relatively good. However, Ni-8YSZ has a problem that unless it is operated in a condition of a sufficiently high S/C ratio, catalytic activity is time-course degraded and the output power density is reduced when internal reforming operation is continued for many hours.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to improve the power generation performance of the solid oxide fuel cell by improving the catalytic activity of the fuel electrode.
Another object of the present invention is to provide the solid oxide fuel cell in which no poisoning by carbon occurs and stable power generation is possible even when internal reforming is performed under a condition of a low S/C ratio.
And yet, another object of the present invention is to provide the solid oxide fuel cell in which the time course degradation of the fuel electrode is less when internal reforming is performed and which has excellent durability and reliability.
To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, in a solid oxide fuel cell having the electrolyte electrode assembly in which the fuel electrode is connected to one face of the first solid electrolyte showing oxide ion conductivity and in which the air electrode is connected to the other face, the fuel electrode comprises a cermet of the catalyst and of the second solid electrolyte whose oxide ion conductivity is more than 0.2 S/cm at 1000xc2x0 C.
If a material having high oxide ion conductivity is used as the second solid electrolyte forming part of the fuel electrode, more oxide ions are supplied to the triple phase boundary of the fuel electrode and a cell reaction is promoted. Hence, even if internal reforming is performed under a condition of a low S/C ratio, a reforming reaction proceeds independently through water generated by the cell reaction and the deposition of carbon on the fuel electrode is inhibited.
Moreover, the fuel electrode using the second solid electrolyte having high oxide ion conductivity has higher catalytic activity and less time course degradation than conventional fuel electrodes using YSZ. Thus, if this is used as the fuel electrode of the internal reforming type SOFC, the cell performance, durability, and reliability of SOFC are improved.
Additional objects and advantages of the invention are set forth in the following description, are obvious from the description, or may be learned by practicing the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the claims.